Blue light emitting diode formed in silicon carbide

ABSTRACT

The present invention comprises a light emitting diode formed in silicon carbide and that emits visible light having a wavelength of between about 475-480 nanometers, or between about 455-460 nanometers, or between about 424-428 nanometers. The diode comprises a substrate of alpha silicon carbide having a first conductivity type and a first epitaxial layer of alpha silicon carbide upon the substrate having the same conductivity type as the substrate. A second epitaxial layer of alpha silicon carbide is upon the first epitaxial layer, has the opposite conductivity type from the first layer, and forms a p-n junction with the first epitaxial layer. In preferred embodiments, the first and second epitaxial layers have carrier concentrations sufficiently different from one another so that the amount of hole current and electron current that flow across the junction under biased conditions are different from one another and so that the majority of recombination events take place in the desired epitaxial layer.

FIELD OF THE INVENTION

The present invention relates to the structure and manufacture of lightemitting diodes and in particular relates to light emitting diodesformed in silicon carbide which emit blue light.

BACKGROUND OF THE INVENTION

Light emitting diodes, commonly referred to as "LED's", aresemiconductor devices which convert electrical power into emitted light.

As is known to those familiar with atomic and molecular structure andthe electronic transitions of electrons according to the theories andrules of quantum mechanics, when electrons make transitions betweentheir allowed energy levels in atoms or molecules, these transitions arealways accompanied by a gain or loss of specific quanta of energy.Specifically, raising electrons to higher energy levels absorbs energyand the movement of electrons from higher energy levels to lower onesgenerates energy. Generally, the energy given off when electrons makesuch "downward" transitions is in the form of vibrational energy, oftenobserved as heat, or light energy--i.e. photons--which, if in thevisible spectrum, can be detected by the human eye.

In a light emitting diode, the generation or "injection" of a current ofeither electrons or holes across the diode junction followed by therecombination of the injected carriers with either holes or electronsencourages such electronic transitions and is likewise accompanied byeither vibrational energy or light, or both. In general, transitions indirect band gap materials generate mostly light while transitions inindirect materials generate mostly heat and some light. Direct band gapmaterials are defined as those in which the minimum in the conductionband corresponds to the maxima in the valence band, at the samemomentum. Correspondingly, in indirect band gap materials, therespective minima and maxima do not coincide at the same momentum.

As is further known to those familiar with electronic transitions, thewavelength of the light which is generated is directly related to thesize of the electronic transition. The nature of electromagneticradiation is such that smaller electronic transitions within the visiblerange tend to emit longer wavelengths of light, toward the red portionof the visible spectrum, and that larger energy transitions tend to emitshorter wavelengths of light, toward the violet portion. Furthermore,such transitions are always specifically characteristic of the materialsin which they occur so that the entire field of spectroscopy is basedupon the premise that atoms, molecules and substances can be identifiedby the characteristic manner in which they react with theelectromagnetic spectrum, including visible, ultraviolet and infraredlight. Accordingly, the colors which any given semiconductor materialcan generate are limited and in particular, it has been difficult todate to successfully produce LED's which emit characteristic bluecolors. Because blue is one of the primary colors, the lack of suchconsistently available efficient blue LED's raises problems in a numberof technological fields. Absent available blue light, the colors thatcan be produced or imaged using LED's are limited to red and green andthose colors which can be formed therefrom.

In order to produce blue light, a semiconductor material must have aband gap larger than 2.6 electron volts (eV). As is known to thosefamiliar with semiconductor materials, the band gap represents theenergy difference between the conduction band and the valance band ofthe particular semiconductor material. At the present time, commerciallyavailable visible light emitting diodes based on materials such asgallium phosphide (GaP) or gallium arsenide (GaAs) are not suitable forproducing blue light because the band gaps are on the order of about2.26 eV or less. Instead, a blue light emitting solid state diode mustbe formed from a relatively large gap semiconductor such as galliumnitride (GaN), zinc sulfide (ZnS), zinc selenide (ZnSe) and alphasilicon carbide (also characterized as "hexagonal" or "6H" siliconcarbide). Accordingly, a number of investigators have attempted toproduce blue light emitting diodes using alpha silicon carbide.

Silicon carbide offers a number of advantages as a potentialsemiconductor material for blue light emitting diodes. In particularsilicon carbide can be readily doped both p and n type. In addition toits wide band gap, silicon carbide also has a high thermal conductivity,a high saturated electron drift velocity, and a high breakdown electricfield. To date, however, silicon carbide has not reached the fullcommercial position in the manufacture of electronic devices, includinglight emitting diodes, that would be expected on the basis of itsexcellent semiconductor properties and its potential for producing blueLED's. This is generally the result of the difficulties encountered inworking with silicon carbide: high process temperatures are oftenrequired, good starting materials can be difficult to obtain, certaindoping techniques have heretofore been difficult to accomplish, andperhaps most importantly, silicon carbide crystallizes in over 150polytypes, many of which are separated by very small thermodynamicdifferences.

Accordingly, the goal of controlling the growth of single crystals ormonocrystalline thin films of silicon carbide which are of a sufficientquality to make electronic devices such as diodes practical, useful andcommercially viable, has eluded researchers in spite of years ofdiligent effort, much of which is reflected in both the patent andnonpatent literature.

Recently, however, a number of developments have been accomplished whichoffer the ability to grow large single crystals of device qualitysilicon carbide, thin films of device quality silicon carbide, and tointroduce dopants to silicon carbide, as required in the manufacture ofLED's and other electronic devices. These development are the subject ofco-pending patent applications which have been assigned to the commonassignee of the present invention and which are incorporated herein byreference. These include Davis et al, "Growth of Beta-Sic Thin Films andSemiconductor Devices Fabricated Thereon," Ser. No. 113,921, Filed Oct.26, 1987; Davis et al, "Homoepitaxial Growth of Alpha-Sic Thin Films andSemiconductor Devices Fabricated Thereon," Ser. No. 113,573, Filed Oct.26, 1987; Davis et al, "Sublimation of Silicon Carbide to Produce Large,Device Quality Single Crystals of Silicon Carbide," Ser. No. 113,565,Filed Oct. 26, 1987; and Edmond et al, "Implantation and ElectricalActivation of Dopants Into Monocrystalline Silicon Carbide," Ser. No.113,561, Filed Oct. 26, 1987.

The alpha polytype of silicon carbide has a band gap of 2.9 electronvolts at room temperature. This band gap is large enough so that anycolor in the visible spectrum should be available providing theappropriate transition can be made. Because the transition is 2.9 eV inpure silicon carbide, however, a full band gap transition produces lightof 424-428 nanometers (nm) wavelength, which has a characteristic violetcolor. Therefore silicon carbide typically must be doped to provide anadditional acceptor level in the crystal to which electrons can movefrom the conduction band of silicon carbide. For example, if siliconcarbide is doped with aluminum, the aluminum dopant will form anacceptor level which is approximately 2.7 eV below the conduction band.As a result, electrons making the transition from the conduction band ofsilicon carbide to the aluminum dopant acceptor level will emit bluelight of approximately 455-460 nanometers.

As set forth earlier, because light is emitted by electrons intransition between energy level, the goal in producing a light from asemiconductor device is promoting or otherwise encouraging suchtransitions. A diode, which reduced to its basic structure represents ap-n junction, such a method for encouraging the transitions. When holesor electrons are injected across the p-n junction, they will recombinewith one another, and a number of the recombination events will includethe movement of electrons from conduction or donor bands to valance oracceptor bands and emit the desired light.

Because the overall goal in producing LED's is to obtain as much emittedlight as possible the underlying related goals are to be able to injectas much current as possible across the p-n junction, to have thegreatest possible dopant population in the emitting layer, to have thegreatest possible efficiency in producing recombination events, and tohave a physical structure, including transparency, which enhances thevisible light obtained from the diode.

In this regard, the flow of current in a diode can be thought of eitheras the flow of electrons from n to p or the flow of holes from p to n.To obtain various hues of blue emitting devices, both modes of injectionare necessary. In most instances, however, it is desireable for most ofthe current going through the diode to be n-type current; i.e. the flowof electrons across the junction and into the p-type material. Thisresults in a higher electron population on the p side of the junctionand a resulting greater number of recombinations, transitions, andphotons emitted.

Presently available blue LED's, however, tend to operate on a lessfavorable basis. One particular commercially available device uses thehigher p current in order to attempt to get the desired number ofrecombinations and in particular uses a p+-n junction in which, asexplained more fully hereinafter, the "+" designation represents agenerally greater population of active dopant in the particularmaterial. Such a device works predominantly on hole injection to get therecombination which results in the 480 nanometer emission.

As stated earlier, however, the full band gap in silicon carbide isapproximately 2.9 eV, and a transition across this band gap will producea violet photon rather than a blue one. The most efficient transition insilicon carbide, however, is between an impurity band of nitrogen(donor) below the conduction band and an impurity band of aluminum(acceptor) above the valence band so that electrons and holes whichrecombine upon injection make a transition between the doped nitrogenand doped aluminum bands and emit the 475-480 nanometer photon which hasa more characteristic blue color. Therefore, the predominant carrierflow or injection--whether electrons or holes--must be made into thecompensated layer, whether p or n. As a result, in order to use holecurrent to produce blue light, the portion of the diode which is n-typemust be doped with both donor (nitrogen) and acceptor (aluminum)dopants, a technique and structure known as "compensation. " Therefore,in order to have a compensated n-type material, a greater number ofn-type dopant atoms must be present in the material than p-type dopantatoms.

As stated earlier, one commercially available LED uses a p+-n junctionof this type to get the 480 nanometer recombination and resultingphoton. Such LED's are formed by using a p-type substrate and growing ap+ layer on top by liquid phase epitaxy (LPE) with aluminum (Al) as thep-type dopant. Following the addition of the p+ layer during LPE,nitrogen gas may be bubbled into the LPE melt. With the aluminum dopantsstill in place, the result is a compensated n-type layer. By using thisgrowth technique, one is essentially limited to this deviceconfiguration.

There are a number of problems and limitations, however, associated withthe use of liquid phase epitaxy to form a p+-n junction. First, itrequires the use of a p substrate. Generally, such a substrate has arather high resistivity because the mobility of holes is only one-sixthof the mobility of electrons and because less than 2% of the acceptoratoms are ionized at room temperature. This results in a higherresistance in forward bias for a diode, which as known to those familiarwith such devices, is a less desireable diode characteristic.

One "cure" for this problem is to increase the hole concentration in thep-type substrate. The addition of the extra dopant, however, literallymakes the crystal opaque and reduces the emitted light that can beobserved. Thus, the problem is a trade off between desirable hightransparency and undesirable high resistivity. Adding more p-type dopantto desirably lower the resistivity correspondingly and undesirablylowers the transparency. Alternatively, maintaining a desirabletransparency correspondingly and undesirably results in highresistivity.

Yet another attempt to avoid the problem is to put both contacts for thediode on the face of the diode in order to avoid using the substrate asa conductor; see, for example, U.S. Pat. No. 4,531,142. This is anextremely difficult manufacturing technique, however, as reflected inthe lower availability and high cost of such diodes.

In addition to using a highly transparent substrate, the light outputcan be increased by increasing the current injected into the compensatedlayer. Here the attempt is to increase the p concentration in the pregion, which requires increasing the p-type dopant in the epitaxiallayer. There are, however, limitations to how high the p concentrationcan made. In particular, every dopant atom present does notautomatically result in an ionized carrier (hole or electron) beingpresent. Generally speaking, the amount of ionized carriers is directlyproportional to the number of dopant atoms, but is inversely andexponentially proportional to the ionization (activation) energy of thedopant atom. For example, the ionization energy of aluminum is on theorder of 210-220 millielectron volts (meV), while that of nitrogen isonly 70-80 meV. Therefore, it is much easier to raise the concentrationof ionized n-type dopant atoms than it is to raise the concentration ofionized p-type dopant atoms using nitrogen and aluminum respectively.

Those familiar with such transitions will be aware that the ionizationof the materials is thermally generated; i.e. the number of dopant atomsionized depends upon the temperature as well as the ionization energy.For example, at room temperature with a doping level of 1×10¹⁹atoms/cm³, only approximately 1% of aluminum carrier atoms are ionizedwhile approximately 22% of nitrogen carrier atoms are ionized.Therefore, for the same number of dopant atoms, the population ofionized n-type dopant ions will be many times as great as that of p-typedopant atoms. As a corresponding result, adding more p-typedopant--usually aluminum--to lower the resistivity likewise lowers thetransparency. Furthermore, obtaining a satisfactory p+ layer at roomtemperature is difficult, as the upper limit of ionized carrierconcentration is about 1 to 2×10¹⁸ cm⁻³ at room temperature. As anotherdisadvantage, the manufacturing technique will always be limited by thesolid solubility of aluminum in silicon carbide.

Furthermore, LPE processes tend to facilitate the addition of n layersto p layers rather than vice versa because nitrogen gas can beintroduced into the melt to add the dopant to the resulting epitaxiallayer. A typical acceptor atom such as aluminum, however, is much moredifficult to add to an epitaxial layer in LPE processes than isnitrogen. Accordingly, adding a p-type epitaxial layer to an n-typesubstrate is generally regarded as an unavailable process for formingdiodes in silicon carbide. The reason is that instead of being able tointroduce nitrogen gas into the melt to form the n-type layer, aluminumwould have to be introduced as the last step, a process which is muchmore difficult, if not impossible, compared to adding nitrogen gas.

Finally, LPE processes for SiC comprise growth of epitaxial layers froma silicon melt in a graphite crucible at temperature in excess of 1600°C. Typically, impurities in the graphite crucible, which is physicallyconsumed during epitaxial growth as part of the LPE process, becomeincorporated in the growing epitaxial layers; i.e. the p+ and ncompensated layers. Many of these impurities have energy levels thatfall within the SiC band gap and their presence leads additionalundesired recombination events, resulting photons, and a consequentbroadening of the emission peak. Therefore, using this growth technique,sharp, narrow bandwidth emission has not been demonstrated to bepractical. For example, the aforementioned commercially available LEDspecifies a full width at half maximum (FWHM) bandwidth (also referredto as "spectral half-width") of 90-95 nanometers.

Accordingly, there exists the need for improved techniques formanufacturing, and improved resulting structures of, blue LED's formedin silicon carbide that can operate on the basis of injection ofelectrons as well as holes, that can therefore achieve higher dopantconcentrations, higher purity films, more transparent substrates, bettercurrent-voltage characteristics, lower resistance, and that can be usedto produce diodes which emit in the 475-480 nanometer range, in the455-460 nanometer range, and in the 424-428 nanometer range, all withnarrow bandwidths.

SUMMARY OF THE INVENTION

Accordingly, the present invention comprises a light emitting diodeformed in silicon carbide and that emits visible light having awavelength of between about 475-480 nanometers, or between about 455-460nanometers, or between about 424-428 nanometers. The diode comprises asubstrate of alpha silicon carbide having a first conductivity type anda first epitaxial layer of alpha silicon carbide upon the substratehaving the same conductivity type as the substrate. A second epitaxiallayer of alpha silicon carbide is upon the first epitaxial layer, hasthe opposite conductivity type from the first layer, and forms a p-njunction with the first epitaxial layer. In preferred embodiments, thefirst and second epitaxial layers have carrier concentrationssufficiently different from one another so that the amount of holecurrent and electron current that flow across the junction under biasedconditions are different from one another and so that the majority ofrecombination events take place in the desired epitaxial layer.

The foregoing and other objects, advantages and features of theinvention, and the manner in which the same are accomplished, willbecome more readily apparent upon consideration of the followingdetailed description of the invention taking in conjunction with theaccompanying drawings, which illustrate preferred and exemplaryembodiments, and wherein:

DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are schematic diagrams of diodes according to the presentinvention that emit at a peak wavelength of between about 455 and 460nanometers;

FIGS. 3 and 4 are schematic diagrams of diodes according to the presentinvention that emit at a peak wavelength of between about 424 and 428nanometers;

FIGS. 5 through 8 are schematic diagrams of diodes according to thepresent invention that emit at a peak wavelength of between about 475and 480 nanometers;

FIGS. 9 and 10 are emission spectra for diodes according to the presentinvention;

FIG. 11 is a plot of current versus voltage characteristics for a diodeaccording to the present invention; and

FIG. 12 is a cross-sectional view of a mesa LED structure according tothe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a light emitting diode generally designated at 20 formed insilicon carbide according to the present invention and which emitsvisible light having a wavelength of between about 455-460 nanometers.The diode comprises an n-type substrate of alpha silicon carbide 21. Anohmic contact 22 provides electrical contact to the substrate 21. Inparticular embodiments of the invention, the ohmic contact 22 to then-type substrate 21, or to n-type epitaxial layers as describedhereinafter, may comprise a metal such as nickel. Similarly, the ohmiccontacts to be described hereinafter to both p-type substrates andp-type epitaxial layers may comprise a metal such as aluminum.

In the diode 20, a substantially uncompensated n-type epitaxial layer 23is upon the n-type substrate 21 and has a carrier concentration greaterthan the carrier concentration of the n-type substrate. As used herein,the phrase "substantially uncompensated" refers to a crystallineepitaxial structure in which some minor amount of opposite conductivitytype dopant may be present, but not in an amount which gives the crystalthose characteristics which those familiar with such structuresgenerally refer to as "compensated," and which will be more fullydescribed hereinafter. As mentioned earlier herein, the greater donorconcentration in the epitaxial layer 23 is often symbolized by adesignation such as "n+". A substantially uncompensated p-type epitaxiallayer 24 of alpha-type silicon carbide is upon the n+-type epitaxiallayer 23 and forms a p-n junction with the n+-type layer. The p-typeepitaxial layer 24 has a carrier concentration less than the carrierconcentration of the n+-type epitaxial layer 23.

As used herein when referring to the populations of the respectiveepitaxial layers of the diodes described herein, the designations"greater than" or "less than" refer to a difference in carrierconcentration between the respective epitaxial layers that is largeenough to cause either the flow of hole current or the flow of electroncurrent to clearly predominate across the junction and so as to likewisecause the majority of recombination events which take place as currentflows across the junction to take place in the desired epitaxial layer.The flow of current through the diode can be thought of as a flow ofelectrons from the n side of the junction to the p side, a current whichis called electron current and represented by the designation "I_(n) ".The current can also be thought of as the flow of holes from p materialacross the junction into the n-type material, a current which isdesignated "I_(p) ".

As further known to those familiar with such devices and theiroperations, I_(n) and I_(p) do not have to be the same and are usuallydifferent. In FIG. 1, the predominate flow is of electron current and isdesignated by the arrow I_(n).

An ohmic contact 25 is made to the substantially uncompensated p-typeepitaxial layer 24 and the resulting diode produces a peak emission at awavelength of between about 455-460 nanometers with a spectralhalf-width at peak wavelength of no more than about 50 nanometers.

In a preferred embodiment, the light emitting diode of FIG. 1 isproduced by growing the respective epitaxial layers by the chemicalvapor deposition technique referred to earlier in co-pending applicationSer. No. 113,573. This application is incorporated entirely herein andthe chemical vapor deposition techniques disclosed therein will bereferred to hereinafter and in the claims for purposes of definition andclarity as "Davis-type" chemical vapor deposition.

Because of the purity and crystalline quality of the epitaxial layersand resulting diodes that result from Davis-type chemical vapordeposition, light emitting diodes such as the one illustrated in FIG. 1can, in preferred embodiments produce a peak emission at the wavelengthof between about 455-460 nanometers with a spectral half-width at peakwavelength of no more than about 25 nanometers. In the diode 20, then-type substrate 21 and n-type epitaxial layer 23 can include nitrogenas the donor carrier and the p-type epitaxial layer can include aluminumas an acceptor carrier.

Furthermore, and as is known to those familiar with the crystallinenature and structure of silicon carbide, the alpha-type silicon carbideused in the substrate and the epitaxial layers can have a polytypeselected from the group consisting of the 6H, 4H, and 15R polytypes.

As will be recognized by those familiar with the operation of suchdiodes, because the p-type layer 24 is doped with aluminum, when apredominating flow of electrons travels from the n+ epitaxial layer 23into the p-type epitaxial layer 24, the predominate recombination eventwill take place between the conduction band of silicon carbide and thedopant band of aluminum. The energy represented by this transmissioncorresponds to the 455-460 nanometer photon and the characteristic bluelight. As stated earlier, there will be a corresponding nonpredominatingflow of holes from the p-type epitaxial layer 24 into the n+ epitaxiallayer 23 which will likewise result in some recombination events such asthose between the conduction and the valance band of silicon carbide,which corresponds to a 424-428 nanometer emission, but because theelectron current predominates, the 455-460 nanometer events and emissionof light will likewise predominate.

As set forth above, the carrier concentration of the n+ layer 23 ismaintained sufficiently greater than the carrier concentration of thep-type epitaxial layer 24 to cause the electron current to predominate.Typically, the carrier concentration of such a "greater than" layer willbe at least an order of magnitude greater than the adjacent layer intowhich the carriers are being injected. In a typical diode, the substratewould have a donor level in the range of about 5×10¹⁷ to 1×10¹⁸ carriersper cubic centimeter (often abbreviated cm⁻³), while in an n+ epitaxiallayer the donor population would be on the order of between about 5×10¹⁸and 2×10¹⁹ cm⁻³. In such a situation, the p-type epitaxial layer willhave a hole population of approximately 2×10¹⁷ to 2×10¹⁸ cm⁻³. A typicalsize for such a diode is a substrate thickness on the order of about 300microns, with epitaxial layers on the order of one or two microns as maybe desired to accomplish various characteristics of the diode.

FIG. 2 is a schematic diagram of another light emitting diode 30 formedin silicon carbide on a p-type substrate 31 of alpha-type siliconcarbide. An ohmic contact 32 is provided to the substrate 31 and asubstantially uncompensated p-type epitaxial layer 33 is upon the p-typesubstrate 31. A substantially uncompensated n-type epitaxial layer 34 ofalpha silicon carbide is upon the p-type epitaxial layer 33 and forms ap-n junction with the p-type epitaxial layer 33. The n-type epitaxiallayer 34 has a carrier concentration greater than the carrierconcentration of the p-type epitaxial layer 33, and an ohmic contact 35is formed to the n-type epitaxial layer. The diode 30 likewise producesa peak emission at the wavelength of between about 455-460 nanometerswith a spectral half-width at peak wavelength of no more than about 50nanometers.

In a preferred embodiment, the diode 30 is likewise formed byrespectively growing the epitaxial layers 33 and 34 by Davis-typechemical vapor deposition which in preferred embodiments results in adiode that produces the peak emission at the wavelength of between about455-460 nanometers with a spectral half-width at peak wavelength on nomore than about 25 nanometers.

As in the other embodiments, the n-type epitaxial layer can includenitrogen as a donor carrier and the p-type epitaxial layer and thep-type substrate may both include aluminum as an acceptor carrier. Thediode may likewise be formed from alpha-type silicon carbide selectedfrom the group consisting of the 6H, 4H and 15R polytypes.

As indicated by the arrow I_(n) in FIG. 2, the predominate flow ofcarriers is the flow of electrons and takes place from the n+ layer 34into the p layer 33 resulting in the identical transition set forth withrespect to FIG. 1.

FIGS. 3 and 4 illustrate diodes according to the present invention whichwill emit light having a peak emission of between about 424-428nanometers. Light of this wavelength represents the full band gaptransition between the conduction band and the valance band of siliconcarbide, rather than a transition between one band of silicon carbideand one dopant band, or between two dopant bands. As will be seen withrespect to the structure of FIGS. 3 and 4, this transition isaccomplished by forming the diode with such a structure so that thepredominant current flow is the flow of holes. As is known to thosefamiliar with silicon carbide and its characteristics, the full band gaprecombination event is characteristically preferred in silicon carbideover the transition which would otherwise take place between the n-type(usually nitrogen) dopant level in the n-type material and the valanceband. FIG. 3 illustrates a first type of light emitting diode which willemit the characteristic 424 nanometer photon. The diode is generallydesignated at 40 and includes an n-type substrate 41 of alpha siliconcarbide and an ohmic contact 42 to the n-type substrate 41. Asubstantially uncompensated n-type epitaxial layer 43 of alpha siliconcarbide is upon the n-type substrate 41 and a substantiallyuncompensated p-type epitaxial layer 44 of alpha silicon carbide is uponthe n-type epitaxial layer 43 and forms a p-n junction with the n-typeepitaxial layer. The p-type epitaxial layer 44 has a carrierconcentration greater than the carrier concentration of the n-typeepitaxial layer 43, with both the n-type layer 43 and the p-type layer44 being formed by Davis-type chemical vapor deposition. An ohmiccontact 45 is made to the p-type epitaxial layer 44. As describedearlier, because the p-type epitaxial layer 44 predominates in carrierconcentration, the predominate flow of current will be hole current fromthe p-type epitaxial layer to the n-type epitaxial layer 43 as indicatedby the arrow I_(p). The diode thus produces a peak emission of betweenabout 424-428 nanometers with a spectral half-width at peak wavelengthof no greater than about 50 nanometers. In preferred embodiments, thespectral half-width at peak wavelength is no greater than about 25nanometers.

As in the earlier embodiments, the n-type substrate and the n-typeepitaxial layer 41 and 43 respectively can include nitrogen as a donorcarrier and the p-type epitaxial layer 44 may include aluminum as theacceptor carrier. The substrate 41 and the respective epitaxial layers43 and 44 can have a polytype selected from the group consisting of 6H,4H and 15R polytypes of silicon carbide, and as likewise in the earlierembodiments, the ohmic contact to the p-type epitaxial layer 44 cancomprise aluminum, and the ohmic contact to the n-type substrate 41 cancomprise nickel. As further set forth with respect to this and earlierembodiments, the n-type substrate 41 provides the advantage of a greatertransparency at a lower resistivity than do the p-type substrates, eventhough the p-type substrates may be preferred under certaincircumstances.

FIG. 4 shows another embodiment of a light emitting diode designated at50 formed on a p-type substrate 51 of alpha silicon carbide. An ohmiccontact 52 is made to the substrate 51 and a substantially uncompensatedp-type epitaxial layer 53 is upon the p-type substrate 51 and has acarrier concentration greater than the carrier concentration of thep-type substrate 51. A first substantially uncompensated n-typeepitaxial layer 54 of alpha silicon carbide is upon the p-type epitaxiallayer 53 and has a carrier concentration less than the carrierconcentration of the p-type epitaxial layer 53. A second substantiallyuncompensated n-type epitaxial layer 55 is upon the first n-typeepitaxial layer 54 and has a carrier concentration greater than thecarrier concentration of the first n-type epitaxial layer 54.

For the diodes illustrated in FIGS. 3 and 4, the p or n substrate wouldtypically have a carrier concentration of between about 5×10¹⁷ and1×10¹⁸ cm⁻³, the p+ epitaxial layer would have a carrier concentrationof between about 1 and 2×10¹⁸ cm⁻³, and the n epitaxial layer would havea carrier concentration of between about 1×10¹⁶ and 1×10¹⁷ cm⁻³. As inthe case of the diodes described with respect to FIGS. 1 and 2, atypical substrate will be on the order of 300 microns thick, with theepitaxial layers being on the order of 1 or 2 microns thick depending onthe desired characteristics and design. It will be understood that theseare typical values and that the diodes of the present invention are notlimited to these specific values.

An ohmic contact 56 is made to the second n-type epitaxial layer 55. Themore highly populated second n-type epitaxial layer 55 thus forms aconductive surface that moderates the current crowding that wouldotherwise occur about the contact 56 to the second n-type epitaxiallayer 55. The resulting diode produces a peak emission of between about424-428 nanometers with a spectral half-width at peak wavelength nogreater than about 50 nanometers.

As indicated by the arrow I_(p) in FIG. 4, the predominating currentflow is the flow of holes from the p-type epitaxial layer 53 into then-type epitaxial layer 54 which, as explained earlier, tends topreferably result in the full band gap transition between the conductionband and valance band of silicon carbide and the emission of theresulting 424-428 nanometer photon.

As in the earlier embodiments, the epitaxial layers 53 and 54 arerespectively grown by Davis-type chemical vapor deposition and in suchpreferred embodiments, the spectral half-width at the peak wavelength isno greater than about 25 nanometers. Similarly to the other embodiments,the donor carrier can include nitrogen and the acceptor carrier caninclude aluminum. The polytype of the substrate and epitaxial layers canbe selected from the group consisting of the 6H, 4H and 15R polytypes ofalpha silicon carbide and the ohmic contacts can comprise nickel to then-type epitaxial layer 55 and aluminum to the p-type substrate 51respectively.

FIG. 5 shows another light emitting diode generally designated at 60formed in silicon carbide according to the present invention and whichemits visible light having a wavelength of between about 475-480nanometers. The diode comprises an n-type substrate of alpha-typesilicon carbide 61. An ohmic contact 62 provides electrical contact tothe substrate 61. A substantially uncompensated n-type epitaxial layer63 of alpha-type silicon carbide is formed upon the n-type substrate 61and has a donor population greater than the donor population of then-type substrate. A compensated p-type epitaxial layer 64 of alpha-typesilicon carbide is formed upon the substantially uncompensated n-typeepitaxial layer and forms a p-n junction with the n-type epitaxial layer63. A substantially uncompensated p-type epitaxial layer 65 ofalpha-type silicon carbide is formed upon the compensated p-typeepitaxial layer and has an acceptor population greater than or equal tothe acceptor population of the compensated p-type epitaxial layer 64. Aconductive contact 66 to the substantially uncompensated p-typeepitaxial layer completes the diode. The purpose of the p+-typeuncompensated epitaxial layer 65 is to form a highly conductive surfacethat evens out the light emitted by the diode and avoids the currentcrowding problems observed in earlier diodes in silicon carbide. It willbe understood, however, that in the absence of the p+ uncompensatedepitaxial layer, a suitable diode can be formed by adding the contact 66directly to the compensated p-type epitaxial layer 64.

As set forth previously, the term "compensated" (which some referencescall "overcompensated") refers to the use of both donor and acceptortype dopants in a doped portion of material which nevertheless maintainseither a donor or an acceptor type characteristic. For example, in acompensated p-type layer, both p-type and n-type dopants (acceptor anddonor atoms respectively) are included, but with the number of p-typeacceptor atoms being sufficiently greater than the number of n-typedonor atoms to give the epitaxial layer p-type characteristics. In asimilar manner, an n-type compensated material would carry both donorand acceptor atoms as intended dopants, but with the number of donortype atoms sufficiently exceeding the number of acceptor type atoms togive the entire epitaxial layer n-type characteristics.

As further set forth earlier herein, the flow of current through thediode can be thought of as a flow of electrons from the n side of thejunction to the p side, a current which is called electron current andrepresented by the designation "I_(n) ". The current can also beidentified as the flow of holes from p material into the n-type materialacross the junction, a current which is designated "I_(p) ". For adevice which emits blue light utilizing electron injection into a p-typecompensated layer, most of the current under forward bias shouldpreferably be electron current (I_(n)) which results in injection of alarge population of electrons into the compensated p side of thejunction, which in turn results in more recombination events and agreater quantity of light emitted. Conversely, for an n-type compensatedlayer, most of the current under forward bias should preferably be holecurrent (Ip) which results in the injection of a large population ofholes into the compensated n side of the junction. In order toaccomplish this, and as set forth in the discussion above, an n+ or p+layer is used to form an n+-p or p+-n junction, respectively; i.e. ajunction in which more carriers (holes or electrons) are present on theside of the junction from which carriers are injected (the "+" side)

For the diodes discussed with respect to FIGS. 5-8, typical carrierconcentrations (cm⁻³) include 5×10¹⁷ to 1×10¹⁸ for the respective n or psubstrates, 5×10¹⁸ to 2×10¹⁹ for the n+ epitaxial layers, 5×10¹⁶ to5×10¹⁷ for the compensated n-type epitaxial layers, 1-2×10¹⁸ for the p+epitaxial layers, and 1×10¹⁶ to 5×10¹⁷ ; for the compensated p-typelayer. As set forth with respect to the other diodes discussed herein,typical substrates can be on the order of 300 microns thick, whiletypical epitaxial layers will be on the order of 1 or 2 microns thick.As stated previously, these are typical values, and the diodes of thepresent invention are not limited to these particular values or rangesof values.

As discussed above with respect to the prior art, devices haveheretofore used only p+-n (compensated) junctions in order to get the475-480 nanometer recombination. From a practical standpoint, theprocess of liquid phase epitaxial growth (LPE) of SiC generally requiresthe use of a p-type substrate and limits design and fabrication to p+-nstructures. More importantly, LPE results in a relatively unpure anddefective compensated n layer which generates an emission with a verybroad bandwidth.

In contrast, the process of Davis-type chemical vapor depositionprovides flexibility to utilize both n or p type substrates and toproduce LED's with narrow-band emission at three various wavelengths.

The advantages of the diode of the invention as just discussed includethe following: (1) a higher injection current due to the ability toobtain a higher donor population in the n+ layer; (2) a highlytransparent, almost colorless substrate with low resistivity; (3) bettercurrent voltage characteristics; (4) high purity in the epitaxial layersgrown by Davis-type CVD; and (5) narrow bandwidth emissions, whichresult from the high purity of the epitaxial layers. One can also takeadvantage of the latter three features and utilize hole injection intoan n compensated layer as shown in FIG. 6. Likewise one can obtainimproved device characteristics by taking advantage of (1) and (4) (seeFIG. 7) or (4) alone (FIG. 8).

In FIG. 6, the diode is designated at 70, the n-type substrate at 71,the ohmic contact to the substrate at 72, the compensated n-typeepitaxial layer at 73, the p+ epitaxial layer at 74, and the ohmiccontact to the p+ layer at 75. As indicated by the arrow I_(p), thediode of FIG. 6 operates by injection of holes from the p+ layer 74 intothe compensated n-type epitaxial layer 73.

In FIG. 7, the diode is designated at 80, the p-type substrate at 81,the ohmic contact to the substrate at 82, the compensated p-typeepitaxial layer at 83, the n+ epitaxial layer at 84, and the ohmiccontact to the n+ layer at 85. In this embodiment, electron currentdesignated by the arrow I_(n) flows from the n+ epitaxial layer 84 intothe compensated p-type epitaxial layer 83.

In FIG. 8, the diode is designated at 90, the p-type substrate at 91,the ohmic contact to the substrate at 92, the p+ epitaxial layer at 93,the compensated n-type epitaxial layer at 94, a substantiallyuncompensated n+ epitaxial layer at 95, and the ohmic contact to the n+layer at 96. As noted with respect to the embodiment of FIG. 4, the n+epitaxial layer 95 provides a highly conductive surface that helpsprevent current crowding that would otherwise occur about the contact96. As indicated by the arrow I_(p), this embodiment operates by theinjection of hole current from the p+ layer 93 into the compensatedn-type epitaxial layer 94.

FIGS. 9 and 10, represent the spectrum of 455 nanometers and 424nanometers LED's respectively, formed according to the present inventionand plotted as units of intensity (relative) against wavelength innanometers. Wavelength data for the diodes represented in FIGS. 9 and 10are summarized in Table I.

                  TABLE I                                                         ______________________________________                                        FIG.      Peak Wavelength                                                                            Spectral Halfwidth                                     ______________________________________                                        9         456 nm       26 nm                                                  10        425 nm       23 nm                                                  ______________________________________                                    

As a brief summary of the technique set forth in the co-pendingapplication Ser. No. 113, 573, the epitaxial layer of the diodes of thepresent invention are grown by chemical vapor deposition of alphasilicon carbide on a lapped and polished, off-axis substrate of alphasilicon carbide. In forming an uncompensated n-type layer, a stream ofethylene (C₂ H₄) and silane (SiH₄) gases are introduced into the CVDsystem. A nitrogen containing gas is introduced at the chosenconcentration and in accordance with the parameters set forth in theincorporated reference, an n-type epitaxial layer of alpha siliconcarbide is formed.

All of the gases are then turned off so that the system is clear,following which ethylene and silane are reintroduced to begin theprocess of forming the p-type layer. Aluminum is added by bubblinghydrogen gas (H₂) through trimethyl aluminum (TMA) to form an aluminumcontaining gas. The result is a p-type epitaxial layer upon the n-typeepitaxial layer both of which have been formed in an essentiallycontinuous process.

In order to form a compensated epitaxial layer, the flow of nitrogen inthe previous process is either maintained or slightly reduced whilealuminum is added as just described, but without stopping the flow ofnitrogen.

In order to form the p+ epitaxial layer upon the compensated layer, theprevious process can be continued but with nitrogen turned off at theend so that only aluminum is added to the last epitaxial layer formed.

FIG. 11 shows the current-voltage characteristics of a diode accordingto the present invention, and in particular shows a forward current of100 milliamps at a forward voltage of 4.4 volts, and a reverse leakagecurrent of less than 1 microamp at -5 volts.

Finally, FIG. 12 illustrates a cross section of a mesa configurationwhich provides a suitable structure for the light emitting diodes of thepresent invention. The substrate is indicated at 101, the respectiveepitaxial layers at 102 and 103, and the ohmic contacts at 104 and 105to the substrate and epitaxial layer 103 respectively. In the mesaconfiguration, a passivation layer 106, for example of silicon dioxide,may be added to protect the p-n junction. Following the growth of theepitaxial layers by Davis-type chemical vapor deposition, the layers andsubstrate may be appropriately etched, and the ohmic contacts andpassivation layer added using otherwise conventional techniques such aphotolithography. A suitable etching technique is set forth in thecopending application of Palmour, "Dry Etching of Silicon Carbide," Ser.No. 116,467, filed Nov. 3, 1987, which is assigned to the assignee ofthe present invention and which is likewise incorporated entirely hereinby reference. As known to those familiar with device manufacture, themesa configuration delineates the p-n junction and isolates junctionsfrom one another when several or many are formed on a common substrate.Accordingly, when a number of such diodes are fabricated at one timeadjacent one another, a usual manufacturing scenario, separating theindividual diodes, usually by sawing the substrate, will not harm theedges or the junction, or otherwise affect the desired crystalstructure.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms havebeen employed, they have been used in a generic and descriptive senseonly and not for purposes of limitation, the scope of the inventionbeing set forth in the following claims.

That which I claim is:
 1. A light emitting diode formed in siliconcarbide that emits light in the blue to violet portion of the visiblespectrum and comprising:an n-type substrate of alpha-type siliconcarbide; an ohmic contact to said substrate; a substantiallyuncompensated n-type monocrystalline epitaxial layer alpha-type siliconcarbide upon said n-type substrate; a p-type monocrystalline epitaxiallayer of alpha-type silicon carbide upon said n-type epitaxial layer andforming a p-n junction with said n-type layer, said p-type epitaxiallayer having a carrier concentration less than the carrier concentrationof said uncompensated n-type epitaxial layer; and an ohmic contact tosaid p-type epitaxial layer, said diode producing a peak emission at awavelength of between about 455 and 460 nanometers with a spectral halfwidth at peak wavelength of no more than about 50 nanometers.
 2. A lightemitting diode according to claim 1 wherein said n-type epitaxial layerhas a carrier concentration greater than the carrier concentration ofsaid n-type substrate.
 3. A light emitting diode according to claim 1that produces a peak emission at a wavelength of between about 455 and460 nanometers with a spectral half width at peak wavelength of no morethan about 25 nanometers.
 4. A light emitting diode according to claim 1wherein said n-type substrate and said n-type epitaxial layer includenitrogen as a donor carrier.
 5. A light emitting diode according toclaim 1 wherein said p-type epitaxial layer includes aluminum as anacceptor carrier.
 6. A light emitting diode according to claim 1 whereinthe alpha-type silicon carbide has a polytype selected from the groupconsisting of: 6H, 4H, and 15R.
 7. A light emitting diode according toclaim 1 wherein said ohmic contact to said n-type substrate comprisesnickel and said ohmic contact to said p-type epitaxial layer comprisesaluminum.
 8. A light emitting diode according to claim 1 wherein saidmonocrystalline n-type epitaxial layer comprises a single polytype andsaid p-type monocrystalline epitaxial layer comprises a single polytype.9. A light emitting diode formed in silicon carbide that emits light inthe blue to violet portion of the visible spectrum and comprising:ap-type substrate of alpha-type silicon carbide; an ohmic contact to saidsubstrate; a substantially uncompensated p-type monocrystallineepitaxial layer of alpha-type silicon carbide upon said p-typesubstrate; a substantially uncompensated n-type monocrystallineepitaxial layer of alpha-type silicon carbide upon said p-type epitaxiallayer and forming a p-n junction with said p-type layer, said n-typeepitaxial layer having a carrier concentration greater than the carrierconcentration of said p-type epitaxial layer; and an ohmic contact tosaid n-type epitaxial layer, said diode producing a peak emission at awavelength of between about 455 and 460 nanometers with a spectral halfwidth at peak wavelength of no more than about 50 nanometers.
 10. Alight emitting diode according to claim 9 that produces a peak emissionat a wavelength of between about 455 and 460 nanometers with a spectralhalf width at peak wavelength of no more than about 25 nanometers.
 11. Alight emitting diode according to claim 9 wherein said n-type epitaxiallayer includes nitrogen as a donor carrier.
 12. A light emitting diodeaccording to claim 9 wherein said p-type substrate and said p-typeepitaxial layer include aluminum as an acceptor carrier.
 13. A lightemitting diode according to claim 9 wherein the alpha-type siliconcarbide has a polytype selected from the group consisting of: 6H, 4H,and 15R.
 14. A light emitting diode according to claim 9 wherein saidohmic contact to said substantially uncompensated n-type epitaxial layercomprises nickel and said ohmic contact to said p-type substratecomprises aluminum.
 15. A light emitting diode according to claim 9wherein said monocrystalline p-type epitaxial layer comprises a singlepolytype and said n-type monocrystalline epitaxial layer comprises asingle polytype.
 16. A light emitting diode formed in silicon carbidethat emits light in the blue to violet portion of the visible spectrumand comprising:an n-type substrate of alpha-type silicon carbide; anohmic contact to said n-type substrate; a substantially uncompensatedn-type monocrystalline epitaxial layer of alpha-type silicon carbideupon said n-type substrate; a substantially uncompensated p-typemonocrystalline epitaxial layer of alpha-type silicon carbide upon saidn-type epitaxial layer and forming a p-n junction with said n-typeepitaxial layer, said p-type epitaxial layer having a carrierconcentration greater than the carrier concentration of said n-typeepitaxial layer; and an ohmic contact to said uncompensated p-typeepitaxial layer, said diode producing a peak emission of between about424 and 428 nanometers with a spectral half width at peak wavelengths ofno greater than about 50 nanometers.
 17. A light emitting diodeaccording to claim 16 that produces a peak emission of between about 424and 428 nanometers with a spectral half width at peak wavelength of nogreater than about 25 nanometers.
 18. A light emitting diode accordingto claim 16 wherein said n-type substrate and said n-type epitaxiallayer include nitrogen as a donor carrier.
 19. A light emitting diodeaccording to claim 16 wherein said p-type epitaxial layer includesaluminum as an acceptor carrier.
 20. A light emitting diode according toclaim 16 wherein the alpha-type silicon carbide has a polytype selectedfrom the group consisting of: 6H, 4H, and 15R.
 21. A light emittingdiode according to claim 16 wherein said ohmic contact to saidsubstantially uncompensated p-type epitaxial layer comprises aluminumand said ohmic contact to said n-type substrate comprises nickel.
 22. Alight emitting diode according to claim 16 wherein said monocrystallinen-type epitaxial layer comprises a single polytype and said p-typemonocrystalline epitaxial layer comprises a single polytype.
 23. A lightemitting diode formed in silicon carbide that emits light in the blue toviolet portion of the visible spectrum and comprising:a p-type substrateof alpha silicon carbide; an ohmic contact to said p-type substrate; asubstantially uncompensated p-type monocrystalline epitaxial layer ofsilicon carbide upon said p-type substrate; a first substantiallyuncompensated n-type monocrystalline epitaxial layer of alpha siliconcarbide upon said p-type epitaxial layer, and having a carrierconcentration less than the carrier concentration of said p-typeepitaxial layer; a second substantially uncompensated n-typemonocrystalline epitaxial layer of silicon carbide upon said firstn-type epitaxial layer, and having a carrier concentration greater thanthe carrier concentration of said first n-type epitaxial layer; and anohmic contact to said second n-type epitaxial layer, wherein said secondn-type epitaxial layer forms a conductive surface that moderates thecurrent crowding that would otherwise occur about said contact to saidsecond n-type epitaxial layer, said diode producing a peak emission ofbetween about 424 and 428 nanometers with a spectral half width at peakwavelength of no greater than about 50 nanometers.
 24. A light emittingdiode according to claim 23 wherein said substantially uncompensatedp-type epitaxial layer has a carrier concentration greater than thecarrier concentration of said p-type substrate.
 25. A light emittingdiode according to claim 23 that produces a peak emission of betweenabout 424 and 428 nanometers with a spectral half width at peakwavelength of no greater than about 25 nanometers.
 26. A light emittingdiode according to claim 23 wherein said n-type epitaxial layers includenitrogen as a donor carrier.
 27. A light emitting diode according toclaim 23 wherein said p-type substrate and said p-type epitaxial layerinclude aluminum as an acceptor carrier.
 28. A light emitting diodeaccording to claim 23 wherein the alpha-type silicon carbide has apolytype selected from the group consisting of: 6H, 4H, and 15R.
 29. Alight emitting diode according to claim 23 wherein said ohmic contact tosaid second n-type epitaxial layer comprises nickel and said ohmiccontact to said p-type substrate comprises aluminum.
 30. A lightemitting diode according to claim 23 wherein said monocrystalline p-typeepitaxial layer comprises a single polytype and said n-typemonocrystalline epitaxial layer comprises a single polytype.
 31. A lightemitting diode formed in silicon carbide that emits light in the blue toviolet portion of the visible spectrum and comprising:an n-typesubstrate of alpha-type silicon carbide; an ohmic contact to said n-typesubstrate; a substantially uncompensated n-type monocrystallineepitaxial layer of alpha-type silicon carbide upon said n-typesubstrate; a compensated p-type monocrystalline epitaxial layer ofalpha-type silicon carbide upon said uncompensated n-type epitaxiallayer and forming a p-n junction with said n-type epitaxial layer; andan ohmic contact to said compensated p-type epitaxial layer.
 32. A lightemitting diode according to claim 31 wherein said uncompensated n-typeepitaxial layer has a carrier concentration greater than the carrierconcentration of said n-type substrate.
 33. A light emitting diodeaccording to claim 31 and further comprising an uncompensated p-typeepitaxial layer of alpha-type silicon carbide upon said compensatedp-type epitaxial layer, said uncompensated p-type epitaxial layer havinga carrier concentration greater than the carrier concentration of saidcompensated p-type epitaxial layer, and wherein said uncompensatedp-type epitaxial layer forms a conductive surface that moderates thecurrent crowding that would otherwise occur about said contact to saidp-type epitaxial layer.
 34. A light emitting diode according to claim 31characterized by the emission of light having a peak wavelength ofbetween about 475 and 480 nanometers with a spectral half width at peakwavelength of no greater than about 50 nanometers.
 35. A light emittingdiode according to claim 31 characterized by the emission of lighthaving a peak wavelength of between about 475 and 480 nanometers with aspectral half width at peak wavelength of no greater than about 25nanometers.
 36. A light emitting diode according to claim 31 whereinsaid n-type substrate and said n-type epitaxial layer include nitrogenas a donor carrier.
 37. A light emitting diode according to claim 31wherein said compensated p-type epitaxial layer includes aluminum as anacceptor carrier and nitrogen as a donor carrier.
 38. A light emittingdiode according to claim 31 wherein the alpha-type silicon carbide has apolytype selected from the group consisting of: 6H, 4H, and 15R.
 39. Alight emitting diode according to claim 31 wherein said ohmic contact tosaid n-type substrate comprises nickel and said ohmic contact to saidcompensated p-type epitaxial layer comprises aluminum.
 40. A lightemitting diode according to claim 31 wherein said monocrystalline n-typeepitaxial layer comprises a single polytype and said p-typemonocrystalline epitaxial layer comprises a single polytype.
 41. A lightemitting diode formed in silicon carbide that emits light in the blue toviolet portion of the visible spectrum and comprising:an n-typesubstrate of alpha silicon carbide; a compensated n-type monocrystallineepitaxial layer of alpha silicon carbide upon said substrate; and asubstantially uncompensated p-type monocrystalline epitaxial layer uponsaid n-type epitaxial layer, said p-type epitaxial layer having acarrier concentration greater than the carrier concentration of saidn-type epitaxial layer; and in which said diode produces a peak emissionat a wavelength of between about 475 and 480 nanometers with a spectralhalf width at peak wavelength of no more than about 50 nanometers.
 42. Alight emitting diode according to claim 41 in which said diode producesa peak emission at a wavelength of between about 475 and 480 nanometerswith a spectral half width at peak wavelength of no more than about 25nanometers.
 43. A light emitting diode according to claim 41 whereinsaid n-type substrate and said compensated n-type epitaxial layerinclude nitrogen as a donor carrier.
 44. A light emitting diodeaccording to claim 43 wherein said compensated n-type epitaxial layerfurther includes aluminum as an acceptor carrier.
 45. A light emittingdiode according to claim 41 wherein said substantially uncompensatedp-type epitaxial layer includes aluminum as an acceptor carrier.
 46. Alight emitting diode according to claim 41 wherein the alpha-typesilicon carbide has a polytype selected from the group consisting of:6H, 4H, and 15R.
 47. A light emitting diode according to claim 41further comprising an ohmic contact to said n-type substrate and anohmic contact to said substantially uncompensated p-type epitaxiallayer.
 48. A light emitting diode according to claim 41 wherein saidmonocrystalline n-type epitaxial layer comprises a single polytype andsad p-type monocrystalline epitaxial layer comprises a single polytype.49. A light emitting emitting diode formed in silicon carbide that emitslight in the blue to violet portion of the visible spectrum andcomprising:a p-type substrate of alpha silicon carbide; a compensatedp-type monocrystalline epitaxial layer of alpha silicon carbide uponsaid susbtrate; and a substantially uncompensated n-type monocrystallineepitaxial layer upon said compensated p-type epitaxial payer and forminga p-n junction with said compensated p-type layer, said n-type epitaxiallayer having a carrier concentration greater than the carrierconcentration of said compensated p-type epitaxial layer.
 50. A lightemitting diode according to claim 49 that produces a peak emission at awavelength of between about 475 and 480 nanometers with a spectral halfwidth of no more than about 50 nanometers.
 51. A light emitting diodeaccording to claim 49 that produces a peak emission at a wavelength ofbetween about 475 and 480 nanometers with a spectral half width of nomore than about 25 nanometers.
 52. A light emitting diode according toclaim 49 wherein said n-type epitaxial layer includes nitrogen as adonor carrier.
 53. A light emitting diode according to claim 49 whereinsaid p-type substrate and said compensated p-type epitaxial layerinclude aluminum as an acceptor carrier.
 54. A light emitting diodeaccording to claim 53 wherein said compensated p-type epitaxial layerfurther includes nitrogen as a donor carrier.
 55. A light emitting diodeaccording to claim 49 wherein the alpha-type silicon carbide has apolytype selected from the group consisting of: 6H, 4H, and 15R.
 56. Alight emitting diode according to claim 49 further comprising an ohmiccontact to said p-type substrate and an ohmic contact to saidsubstantially uncompensated n-type epitaxial layer.
 57. A light emittingdiode according to claim 49 wherein said monocrystalline p-typeepitaxial layer comprises a single polytype and said n-typemonocrystalline epitaxial layer comprises a single polytype.
 58. A lightemitting diode formed in silicon carbide that emits light in the blue toviolet portion of the visible spectrum and comprising:a p-type substrateformed of alpha silicon carbide; an ohmic contact to said substrate; asubstantially uncompensated p-type monocrystalline epitaxial layerformed of alpha silicon carbide upon said substrate; a compensatedn-type monocrystalline epitaxial layer upon said p-type epitaxial layer,said n-type epitaxial layer having a carrier concentration less than thecarrier concentration of said p-type epitaxial layer; and asubstantially uncompensated n-type monocrystalline epitaxial layer uponsaid compensated n-type epitaxial layer and having a carrierconcentration greater than the carrier concentration of said compensatedn-type epitaxial layer; and an ohmic contact to said uncompensatedn-type epitaxial layer, and wherein said n-type epitaxial layer forms aconductive surface that moderates the current crowding that wouldotherwise occur about said contact to said n-type epitaxial layer, andin which said diode produces a peak emission at a wavelength of betweenabout 475 and 480 nanometers with a spectral half width at peakwavelength of no more than about 50 nanometers.
 59. A light emittingdiode according to claim 58 that produces a peak emission at awavelength of between about 475 and 480 nanometers with a spectral halfwidth at peak wavelength of no more than about 25 nanometers.
 60. Alight emitting diode according to claim 58 wherein said compensatedn-type epitaxial layer and said substantially uncompensated n-typeepitaxial layer include nitrogen as a donor carrier.
 61. A lightemitting diode according to claim 60 wherein said compensated n-typeepitaxial layer further includes aluminum as an acceptor carrier.
 62. Alight emitting diode according to claim 58 wherein the alpha-typesilicon carbide has a polytype selected from the group consisting of:6H, 4H, and 15R.
 63. A light emitting diode according to claim 58wherein said ohmic contact to said substantially uncompensated n-typeepitaxial layer comprises nickel and said ohmic contact to said p-typesubstrate comprises aluminum.
 64. A light emitting diode according toclaim 58 wherein said monocrystalline p-type epitaxial layer comprises asingle polytype and said n-type monocrystalline epitaxial layercomprises a single polytype.